Organic solvent type coatings, which are obtained by dissolving main components such as resin and the like in an organic solvent and adding thereto auxiliary components such as coloring agent, curing agent and the like, have heretofore been used widely in coating applications. As there were seriously taken up recently the problems of these organic solvent type coatings, such as fire hazard, adverse effect on safety and hydigene, environmental pollution and the like, attention is being paid to coatings which vaporize no organic solvent, particularly aqueous emulsion type coatings and powder coatings.
Aqueous emulsion type coatings, however, have drawbacks. For example, since, in producing such a coating, resin particles and a pigment must be dispersed stably in an aqueous medium and therefore a hydrophilic substance such as emulsifier or the like is used, the film formed with the coating is inevitably inferior in properties such as water resistance, alkali resistance and the like; moreover, the film has low adhesivity to a material to be coated; further, it takes a time to obtain a dried film, as compared with the case of an organic solvent type coating and, in order to complete film drying in a short time, special equipment and a high cost are required.
Meanwhile, powder coatings, which contain no organic solvent, have various excellent advantages. For example, they are free from fire hazard, adverse effect on safety and hygiene, environmental pollution, or the like; they can be stored in an ordinary storehouse; the air amount of ventillation in spray booth can be minimum and the ventiallation air can be recirculated, resulting in high energy efficiency; and the coating film obtained has no foams which are generated by the vaporization of solvent during film drying. Powder coatings have further advantages. For example, they can be used as. they are without the necessity of adjusting the viscosity, solid content, etc.; and they can be easily recovered without staining the operation site and producing any waste. Furthermore, powder coatings can be applied by automated coating and, in view of the total cost including cost of coating materials, pretreatment cost, cost of coating operation, equipment cost, etc., are very economical as compared with organic solvent type coatings and aqueous emulsion type coatings.
In spite of the above-mentioned various advantages, powder coatings are not used popularly. One reason therefor is that when a conventional powder coating is applied and baked, it is difficult to obtain a coating film having luster and smoothness and, when applied in a thin film, the film cannot hide the substrate thereof sufficiently. In applying an organic solvent type coating or an aqueous coating on a material to be coated, the resulting film can flow during the vaporization of solvent and a smooth and continuous film can be obtained easily; meanwhile, in applying a powder coating, it is difficult to obtain a film having luster and smoothness because the powder (which is a solid) causes fusion bonding and thereby a film is formed. In order to obtain a film having luster and smoothness with a powder coating, it was proposed to make smaller the particle diameters of powder coating. However, this approach is not practical because the smaller particle diameters of powder coating result in lower fluidity of particles (i.e. powder coating) and inferior coatability.
In addition, powder coatings essentially have a serious drawback. Generally, coatings are required to not only have a role of protecting a material to be coated, from outside circumstances but also give a fine appearance on the material to be coated and, therefore, are required to be prepared as a series of many coatings each having a different color. Meanwhile, powder coatings are not suitable for production of many coatings of different colors, owing to the particular production process.
Production of a powder coating is ordinarily conducted by appropriately blending a resin, a curing agent, a fluidity-controlling agent, a coloring agent, a charge control agent, etc., subjecting the resulting blend to heating, melting and kneading by the use of an extruder or the like, and subjecting the kneaded product to cooling, grinding and classification; thus, many steps are required. In order to obtain various powder coatings of different colors, these many steps must be conducted for each of the coatings of different colors. Therefore, in producing many powder coatings of different colors, a large number of steps and a big expense are required, and, moreover, control of various raw materials and control of steps are very complicated.
In powder coatings, when a powder coating of certain color is produced and then a powder coating of different color is produced using the same apparatuses, the latter coating is contaminated with the former coating unless all the apparatuses used in production of the former coating are cleaned sufficiently. Therefore, when sufficient cleaning is difficult, another series of production apparatuses are required for production of the latter coating and, when a number of coatings of different colors are produced, a number of series of production apparatuses are required. This is very disadvantageous as compared with when other types of coatings are produced.
The color of the film formed with a powder coating obtained as above is determined by the kind and amount of coloring agent used in the first production step, the conditions of each step, etc. However, the exact color of such a film can be confirmed only by trial application of a powder coating produced. This requires a number of steps and much time for color matching.
As one method for overcoming the above-mentioned drawbacks of powder coating, it was proposed to mix at least two colored powders each having a different fundamental color and having the maximum particle size of 10 .mu.m or smaller and applying the resulting powder coating to form a film having a desired color (National Publication of International Patent Application No. 504431/1992=WO 60/06345). Meanwhile, a powder coating having particle sizes of smaller than 15 .mu.m is not easily fluidized with, in particular, a commercial electrostatic coating machine. Such a powder which cannot be applied with a commercial electrostatic coating machine, cannot be called a powder coating and is merely a raw material for powder coating. In fact, in the above proposal, a mixture of at least two powder coatings each having a different fundamental color is subjected to particle agglomeration and then is applied as particles of 15-75 .mu.m. In the above proposal, not only an extra step of particle agglomeration is needed, but also cleaning of the agglomeration apparatus is necessary and the above-mentioned problem is still not solved. The above proposal further has a serious drawback in that agglomeration of particles incurs reduction in surface smoothness of applied powder coating layer as well as in thermal conductivity of the layer and, as a result, the coating film after baking is low in smoothness and luster.
The main object of the present invention is to provide a novel powder coating which is free from the above-mentioned problems of conventional powder coatings.
One object of the present invention is to provide a powder coating capable of forming a coating film having smoothness and luster.
Another object of the present invention is to provide a powder coating capable of forming a coating fiIm which is uniform and has a hiding power even when formed in a small thickness.
Still another object of the present invention is to provide a powder coating which can be produced at a high efficiency and is economical.
Still another object of the present invention is to provide a powder coating which makes easy the color matching of different colors and has good coatability.
Other objects of the present invention will become apparent from the following description.